This will execute $cmd in the background (no cmd window) without PHP waiting for it to finish, on both Windows and Unix.

function execInBackground($cmd) {
    if (substr(php_uname(), 0, 7) == "Windows"){
        pclose(popen("start /B ". `$cmd`, "r")); 
    }
    else {
        exec(`$cmd` . " > /dev/null &");  
    }
} ?>

http://php.net/manual/en/function.exec.php